Leaders have previously been chosen by consensus in the group, which brings together diverse regime opponents ranging from liberals to Islamists.

The mandate of current SNC leader Abdel Basset Sayda, which was due to expire on September 9, has been extended and he will stay on until the vote is held in late September, Sabra said.

There were no plans, however, for the SNC to expand to include another prominent opposition grouping, the National Coordination Committee for Democratic Change (NCC), which favours a non-violent overthrow of the regime and opposes foreign military intervention.

"The composition of the SNC will not be the same, but currents close to the NCC will not be integrated," said Monzer Makhous, the SNC's external relations coordinator.

The SNC has faced frequent criticism for failing to fully represent Syria's opposition.

Prominent dissident Bassma Kodmani, a one-time SNC spokeswoman, resigned from the group last week, saying it "does not work well with other opposition groups."